customer n.

c. 1200, custume, "habitual practice," either of an individual or a nation or community, from Old French costume "custom, habit, practice; clothes, dress" (12c., Modern French coutume), from Vulgar Latin *consuetumen, from Latin consuetudinem (nominative consuetudo) "habit, usage, way, practice, tradition, familiarity," from consuetus, past participle of consuescere "accustom," from com-, intensive prefix (see com-), + suescere "become used to, accustom oneself," related to sui, genitive of suus "oneself," from PIE *swe- "oneself" (see idiom).

Custom implies continued volition, the choice to keep doing what one has done; as compared with manner and fashion, it implies a good deal of permanence. [Century Dictionary]

A doublet of costume. An Old English word for it was þeaw. Meaning "the practice of buying goods at some particular place" is from 1590s. Sense of a "regular" toll or tax on goods is early 14c. The native word here is toll (n.).

Custom-house "government office at a point of import and export for the collection of customs" is from late 15c. Customs "area at a seaport, airport, etc., where baggage is examined" is by 1921.

Old customs! Oh! I love the sound,

  However simple they may be:

Whate'er with time has sanction found,

  Is welcome, and is dear to me.

Pride grows above simplicity,

  And spurns it from her haughty mind,

And soon the poet's song will be

  The only refuge they can find.

[from "December," John Clare, 1827]

1775, American English dialectal, "troublesome person or animal" (usually with a defining adjective), a vulgar pronunciation of curse (n.), or else a shortening of the slang sense of customer. The word in the literal sense of "a curse" is from 1848.